1. Field of the Invention
The present invention relates to an improved modular heat exchanger of the type used in the automotive industry. More particularly, this invention relates to a modular heat exchanger having a brazed core assembly and techniques by which such a heat exchanger is assembled, in which the brazed core is manufactured such that the spacial positions of its cooling tubes after brazing enable the tubes to properly mate with elastomerically-sealed apertures formed in the heat exchanger's headers, and thereby form a modular heat exchanger whose brazed core can be readily removed for repair or replacement, and whose overall construction is capable of withstanding structural distortions without leaks and structural failures occurring between the tubes and the headers.
2. Description of the Prior Art
Heat exchangers are routinely employed within the automotive industry, such as in the form of radiators for cooling engine coolant, oil coolers, charge air coolers, condensers and evaporators for air conditioning systems, and heaters. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and a fluid flowing through the heat exchanger, the design of the heat exchanger is often of a brazed tube-and-fin type, such as the tube-and-center construction illustrated in FIG. 1a. A portion of a heat exchanger 10 is shown as having a number of flat-type cooling tubes 12 between pairs of high surface area fins, or centers 14. The centers 14 enhance the ability of the heat exchanger 10 to transfer heat from the fluid flowing through the tubes 12 to the environment, or vice versa as the case may be. The tubes 12 are simultaneously brazed to the centers 14 and a manifold (not shown) to form a monolithic brazed construction defining a fluid circuit.
For reasons of weight and durability, the automotive heat exchanger industry has gradually converted to an aluminum alloy construction. The tubes and centers of such heat exchangers are conventionally formed from an aluminum alloy that is clad with an aluminum-silicon eutectic brazing alloy, such as AA 4045, AA 4047 and AA 4343 aluminum alloys (AA being the designation given by the Aluminum Association), or another suitable brazing alloy, including zinc-base cladding alloys. Such braze alloys have a lower melting temperature than the base aluminum alloy, which is often AA 3003, having a nominal chemistry of about 1.2 weight percent manganese, with the balance being substantially aluminum. The cladding is formed to provide a sufficient amount of braze alloy to produce fluid-tight brazements when the assembled components are heated to a temperature above the melting temperature of the cladding, but below the melting temperature of the base aluminum alloy. Aluminum heat exchanger tubes are generally seamless extruded tubes, preformed welded tubes, or formed from welded strips of aluminum flat stock, with one or both sides of the tube being clad.
While the above-described brazed tube-and-center construction is widely employed in the automotive industry for the manufacture of heat exchangers, certain disadvantages exist. One drawback is that the monolithic brazed construction requires a large drying oven and furnace, both of which are expensive to purchase and operate. Another drawback is that the brazed construction renders such heat exchangers inadequate for more physically demanding applications, such as in the truck and heavy-duty equipment industries. More specifically, monolithic brazed heat exchangers are rigid and therefore do not readily “give” during pressure and thermal cycles, when subject to vibration, or when otherwise distorted by their operating environment. For example, radiators used in large trucks and other large equipment typically have frame mountings that tend to distort the radiator into a parallelogram shape when the vehicle is moving over an uneven surface and when sufficient engine torque is generated. As a result, if a monolithic brazed heat exchanger is used in these applications, cracks eventually develop in the tube-to-header joint where the tubes are brazed to the manifold. Repair of cracks in the tube-to-header joint is expensive, and this mode of failure constitutes a major source of scrappage in the heat exchanger industry. Finally, the working environment of a heavy-duty vehicle employed in construction is severe, leading to a high incidence of damage to the tubes and fins from impacts by debris. Consequently, any localized damage to the core of a monolithic brazed heat exchanger will generally necessitate the removal of the entire heat exchanger for repair or replacement.
The above shortcomings are generally known in the prior art. The response in the heavy duty truck and equipment industries has been to employ a modular heat exchanger construction, such as those represented by U.S. Pat. No. 4,191,244 to Keske, U.S. Pat. No. 4,741,392 to Morse, and U.S. Pat. Nos. 5,289,870 and 5,303,770 to Dierbeck. Such designs employ a modular radiator construction composed of a core and header permanently attached to a manifold or tank. One or more of these self-contained heat exchanger units are then assembled to a common header or tank with the use of grommets, gaskets or other resilient sealing material to form a fluid-tight seal between the module and the common header or tank. Notable examples of grommet designs are taught in U.S. Pat. Nos. 4,756,361, 5,205,354 and 5,226,235 to Lesage, commonly assigned with the present invention.
Prior art modular constructions of the type noted above have found wide use because they are more durable and permit replacement of a damaged heat exchanger module without requiring replacement of the entire heat exchanger assembly. However, prior art modular constructions have been generally unable to adopt the brazed tube-and-center core design described previously, in which the tubes and/or fins are formed from a clad aluminum alloy. During brazing, the core module—composed of the tubes and centers—shrinks as the cladding melts and later resolidifies. This tendency is exacerbated if sinusoidal centers are used, such as those shown in FIG. 1a. As a result, assembling a header or tank to the brazed core is impossible because the tubes are misaligned with apertures formed in the header or tank to receive the tubes.
Consequently, prior art modular heat exchanger designs have been assembled with one or more self-contained heat exchanger modules each formed with a common inlet and outlet, as typified by Keske, Morse and Dierbeck. While such an approach may solve the misalignment problem noted above, the result is a module that is nearly as expensive to manufacture as a conventional monolithic brazed heat exchanger. As a more economical alternative, prior art modular heat exchanger have employed cores with a tube and fin construction such as that illustrated in FIG. 1b. With such designs, round tubes 16 and flat fins 18 are mechanically joined together, such as by expanding the tubes 16, as typified by the patents to Lesage. Unfortunately, mechanical joining methods do not yield a metal-to-metal joint that conducts heat as well as the brazed joints formed with the brazed core design illustrated in FIG. 1a. As a result, heat transfer between the fluid in the tubes 16 and air flowing over the fins 18 is not as efficient in prior art modular constructions as compared to monolithic brazed constructions.
From the above, it is apparent that the prior art is lacking an economical modular heat exchanger design that incorporates a brazed core assembly, and further lacks a method or process by which such a heat exchanger could be manufactured. Yet it is also apparent that it would be desirable to provide a heat exchanger that offers the weight and durability benefits of an aluminum alloy construction, the heat transfer efficiency of a brazed tube-and-center core construction, and an economical modular construction that enables a heat exchanger core to be removed and replaced as necessary. Such a modular construction would preferably incorporate a flexible seal between the heat exchanger core and manifold, enabling distortions to occur in the heat exchanger without the occurrence of failures at the tube-to-header joint.